From US 2006/022292 A1 there is known a junction barrier Schottky (JBS) diode which has a substrate and two or more epitaxial layers, including at least a thin, lightly doped N-type top epitaxial layer, and an N-type epitaxial layer on which the topmost epitaxial layer is disposed. Multiple epitaxial layers support the blocking voltage of the diode, and each of the multiple epitaxial layers supports a substantial portion of the blocking voltage. Optimization of the thickness and dopant concentrations of at least the top two epitaxial layers results in reduced capacitance and switching losses, while keeping effects on forward voltage and on-resistance low.
From US 2009/160008 A1 there are known a semiconductor device that includes an n-type semiconductor substrate and an upper electrode formed on an upper face of the semiconductor substrate and a method of manufacturing the semiconductor device. A p-type semiconductor region is repeatedly formed in the semiconductor substrate in at least one direction parallel to the substrate plane so as to be exposed on an upper face of the semiconductor substrate. The upper electrode includes a metal electrode portion; and a semiconductor electrode portion made of a semiconductor material whose band gap is narrower than that of the semiconductor substrate. The semiconductor electrode portion is provided on each p-type semiconductor region exposed on the upper face of the semiconductor substrate. The metal electrode portion is in Schottky contact with an n-type semiconductor region exposed on the upper face of the semiconductor substrate, and is in ohmic contact with the semiconductor electrode portion.
From JP H07 2265521 A there is known a junction barrier Schottky (JBS) diode with two section emitter regions. The two sections of the emitter regions have different doping concentrations.
A junction barrier Schottky (JBS) rectifier is a hybrid power device, which combines a Schottky and pin diode structure in one device, making use of the advantages of both structures. It has a low on-state resistance and a high blocking capability. Silicon carbide (SiC) based JBS rectifiers are candidates to replace silicon (Si) based pin diodes for high blocking voltages. SiC material properties allow devices with higher voltage rating and higher operating temperatures compared to Si.
A common SiC based JBS rectifier is shown in FIG. 1. It comprises a substrate layer 1, which is made of highly doped n-type silicon carbide. A low-doped n-type silicon carbide layer, which is the drift layer 2 of the device, is formed on the substrate layer 1. Adjacent to the surface of the drift layer on a first main side 4 of the JBS rectifier opposite to the substrate layer 1 there are formed a plurality of p-type emitter regions 3. The first main side 4 of the JBS rectifier, which is the anode side of the device, is covered with a first metal contact layer 5 that forms a Schottky barrier in places where the first metal contact layer 5 contacts the n-type drift layer 2 and that forms an ohmic contact with the p-type emitter regions 3 in places where the first metal contact layer 5 contacts the p-type emitter regions 3. Typically the drift layer 2 is grown epitaxially on a highly doped n-type SiC substrate wafer used as the substrate layer 1.
Depending on the electrical polarity of the voltage between anode and cathode, the Schottky contact either blocks current flow or allows the passage of majority carriers (electrons in n-doped semiconductor material). These two modes correspond with the blocking and on-state operation of the JBS rectifier under normal operating conditions.
The blocking capability of the JBS rectifier is mainly given by the thickness and doping density of the n-doped drift layer. However, as a result of the nature of the Schottky contact, image force lowering at elevated electric field levels at high blocking voltages causes the barrier for electrons to shrink. A pure Schottky barrier diode without p-doped regions will be prone to increasing levels of leakage currents at high reverse bias. The comparatively large number of carriers will entail intensified pair generation during impact ionization. As a result, pure Schottky barrier diodes exhibit a relatively high leakage current and low breakdown voltage. In a JBS rectifier, the p-type emitter regions help to improve this situation. Under reverse bias, a depletion layer develops across the pn-junctions between the p-type emitter regions 3 and the n-type drift layer 2 in the same way as it does in a pin diode. The individual depletion zones around the p-doped emitter regions 3 may eventually connect with each other and close in between two adjacent emitter regions 3 below the Schottky contact. In this way the Schottky contact is effectively protected from a high electric field peak. The combination of Schottky contacts with p-doped emitter regions 3 will therefore reduce leakage currents and allow to reach much higher breakdown voltages compared to pure Schottky barrier diodes.
Given the large on-state voltage drop of unipolar power devices it is also a most important requirement for JBS rectifiers that they can properly handle surge current conditions. In such a failure mode operation, the forward current density in the JBS rectifier can increase up to 1000 A/cm2 to 2000 A/cm2 (that is about 10 to 20 times the on-state current density in normal operation). Due to excessive power loss generation this level cannot be handled without failure by the Schottky diode part alone. At this point, the pin diode sections in the JBS rectifier start to conduct when the forward bias exceeds about 3 V to 4 V. The bipolar regime involves the generation of a carrier plasma consisting of electrons and holes. The pin diode part in the JBS rectifier help to safely handle surge current situations without exceeding thermal limits of the device. Achieving this goal imposes additional, different requirements on the p-doped emitter regions 3 at the JBS anode surface. Controlling the surface field at the Schottky contact to increase the breakdown voltage can be achieved with relatively narrow p-doped emitter regions 3, whereas separations exceeding several microns would compromise the breakdown voltage. The requirement to handle surge current situations calls for strong bipolar emitter action of the p-doped emitter regions 3. The simplest way to satisfy this request is a wide and highly p-doped emitter region 3. Unfortunately, such wide emitter region 3 reduces the anode area available for Schottky contacts and thus leads to higher on-state resistance.
In FIGS. 5 to 10 there are shown current voltage characteristics for different JBS rectifiers. The curves referenced as “conventional drift layer” are the forward current voltage characteristics of common JBS rectifiers as discussed above with different doping concentrations in the p-type emitter regions 3 and with different widths of the emitter regions 3. In FIG. 5 the width of the emitter regions is 6 μm and the doping concentration of the emitter regions is 2·1018 cm−3. In FIG. 6 the width of the emitter regions is 14 μm and the doping concentration in the emitter regions is 2·1018 cm−3. In FIG. 7 the width of the emitter regions is 6 μm and the doping concentration in the emitter regions is 2·1019 cm−3. In FIG. 8 the width of the emitter regions is 14 μm and the doping concentration in the emitter regions is 2·1019 cm−3. In FIG. 9 the width of the emitter regions is 6 μm and the doping concentration in the emitter regions is 2·1020 cm−3. In FIG. 10 the width of the emitter regions is 14 μm and the doping concentration in the emitter regions is 2·1020 cm−3.
As can be seen from FIGS. 5 to 10, all common JBS rectifiers show at low forward currents a region with positive differential resistance in which the current is mainly carried through the Schottky barrier diode parts (unipolar conduction mode). At higher forward currents the current is carried mainly through the pin diode parts (bipolar conduction mode). In the transition from the unipolar conduction mode to the bipolar conduction mode the current voltage characteristics exhibit a region with a negative differential resistance. This transition is known for narrow shorted p-emitters, for example for emitter-shorted insulated gate bipolar transistors (IGBTs) as snap-back phenomenon during turning on of the device.
To increase life-time and robustness of the JBS rectifiers under surge current conditions no or minimal snap-back phenomenon and a transition from unipolar to bipolar conduction mode at a forward bias as low as possible would be preferable.